Organic compounds absorb light to be in an excited state. Organic compounds cause various reactions (such as photochemical reactions) or emit light (luminescence) in some cases through this excited state. Therefore, various applications of the organic compounds have been being made.
As one example of the photochemical reactions, there is a reaction (oxygen addition) of singlet oxygen with an unsaturated organic molecule (for example, see Non-patent Document 1: Haruo INOUE, et al., Basic Chemistry Course PHOTOCHEMISTRY I (published by Maruzen Co., Ltd.), pp. 106-110). Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by direct photoexcitation. Singlet oxygen is generated in the presence of any other triplet excited molecule, which leads to an oxygen addition reaction. A compound which can be the triplet excited molecule is referred to as a photosensitizer.
As described above, a photosensitizer that can make a triplet excited molecule by photoexcitation is necessary in order to generate singlet oxygen. However, since the ground state of an ordinary organic compound is a singlet state, photoexcitation to a triplet excited state is a forbidden transition and a triplet excited molecule is unlikely formed. Therefore, a compound that can easily cause intersystem crossing from a singlet excited state to a triplet excited state (or a compound that allows a forbidden transition and is directly photoexcited to the triplet excited state) is required as a photosensitizer. In other words, such a compound can be used as a photosensitizer and is useful.
The compound often exhibits phosphorescence. Phosphorescence refers to luminescence generated by transition between different energies in multiplicity. In an ordinary organic compound, phosphorescence refers to luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, fluorescence refers to luminescence in returning from the singlet excited state to the singlet ground state). Fields of application of a compound capable of exhibiting phosphorescence, that is, a compound capable of converting the triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light emitting element including an organic compound as a light emitting substance.
Such a light emitting element has a simple structure in which a light emitting layer including an organic compound which is a light emitting substance is provided between electrodes. This light emitting element attracts attention as a next-generation flat panel display element because of its characteristics such as a thin shape, lightweight, high response speed, and direct current low voltage driving. Further, a display device including this light emitting element is superior in contrast, image quality, and has a wide viewing angle.
In a light emitting element including an organic compound as a light emitting substance, an emission mechanism is a carrier injection type. In other words, when voltage is applied between electrodes which sandwich a light emitting layer, electrons and holes are injected from the electrodes and recombined to make the light emitting substance excited, and then, light is emitted when the electrons and holes return from the excited state to the ground state. As in the case of photoexcitation described above, types of the excited state include a singlet excited state (S*) and a triplet excited state (T*). The statistical generation ratio thereof in the light emitting element is considered to be S*:T*=1:3.
A compound capable of converting the singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound) exhibits only luminescence from the singlet excited state (fluorescence), and does not exhibit luminescence from the triplet excited state (phosphorescence) at room temperature. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light emitting element including a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, if the above-described phosphorescent compound is used, the internal quantum efficiency can be improved up to 75 to 100% in theory; that is, the light emission efficiency can be 3 to 4 times as high as that of a fluorescent compound is possible. Therefore, light emitting elements including a phosphorescent compound has been actively developed in recent years in order to realize highly-efficient light emitting elements (for example, see Non-patent Document 2: Chihaya ADACHI, et al., Applied Physics Letters, Vol. 78, No. 11, pp. 1622-1624. (2001)). An organometallic complex that includes iridium or the like as a central metal is particularly attracting attention as a phosphorescent compound because of its high phosphorescence quantum efficiency.